Mixed C4 streams (also called C4 fractions) are obtained in a number of petrochemical processes, see N. Calamur, et al., “Butylenes,” in Kirk-Othmer Encyclopedia of Chemical Technology, online edition, 2007. For example, steam cracking of hydrocarbons is widely used to produce olefins such as ethylene, propylene, butenes (1-butene, cis- and trans-2-butenes, isobutene), butadiene, and aromatics such as benzene, toluene, and xylene. In an olefin plant, a hydrocarbon feedstock such as naphtha, gas oil, or other fractions of whole crude oil is mixed with steam. This mixture, after preheating, is subjected to severe thermal cracking at elevated temperatures (800° C. to 850° C.) in a pyrolysis furnace. The cracked effluent from the pyrolysis furnace contains gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule). This effluent contains hydrocarbons that are aliphatic, aromatic, saturated, and unsaturated, and may contain significant amounts of molecular hydrogen. The cracked product of a pyrolysis furnace is then further processed in the olefin plant to produce, as products of the plant, various individual product streams such as hydrogen, ethylene, propylene, mixed hydrocarbons having four or five carbon atoms per molecule (crude C4's and C5's), and pyrolysis gasoline.
Crude C4's can contain varying amounts of n-butane, isobutane, 1-butene, 2-butene (cis- and/or trans-), isobutene (isobutylene), acetylenes (ethyl acetylene and vinyl acetylene), and butadiene. The term 2-butene as used herein includes cis-2-butene, trans-2-butene, or a mixture of both.
Crude C4's are typically subjected to butadiene extraction or butadiene selective hydrogenation to remove most, if not essentially all, of the butadiene and acetylenes present. Thereafter the C4 raffinate (called raffinate-1) is subjected to a chemical reaction (e.g., etherification, hydration, dimerization) wherein the isobutylene is converted to other compounds (e.g., methyl tertiary butyl ether, tertiary butyl alcohol, diisobutylene) (see, e.g., U.S. Pat. Nos. 6,586,649 and 4,242,530). The remaining C4 stream containing mainly n-butane isobutane, 1-butene and 2-butene is called raffinate-2. Such a stream may react with an isomerization catalyst to produce an isomerized stream with enriched 2-butene as a result of the isomerization of 1-butene. The isomerized butenes stream may further react with ethylene to produce propylene through a so-called metathesis reaction (U.S. Pat. Nos. 5,300,718 and 5,898,091; Appl. Ind. Catal. 3 (1984) 215). One drawback of the process is that the presence of the paraffins (n-butane and isobutane) in the streams limits the catalyst productivity and the throughput of the plant.
It is known that paraffins can be separated from the butenes (1-butene and 2-butene) by extractive distillation or other techniques, see U.S. Pat. Nos. 4,515,661, 5,288,370, U.S. Pat. Appl. Pub. No. 2005/0154246, and DeRosset, A. J., et al., Prepr. —Am. Chem. Soc., Div. Pet. Chem. 23(2) (1978) 766. It would be desirable to remove the paraffins from a feed stream such as a raffinate-2 before the isomerization and the metathesis reaction. Such a paraffins-removal step would significantly increase the throughput of the plant.